1. Field of the Invention
The present invention relates to a CCD (Charge Coupled Device) solid-state imaging device, and more particularly to a solid-state imaging device in which full pixel reading is performed by progressive scanning, and a method of driving the device.
2. Description of Related Art
In the case where signal charges are read out from pixels (photoelectric converting elements) of a CCD solid-state imaging device, as described in, for example, JP-A-7-322143 (Japanese Patent No. 3,509,184) and JP-A-2001-16510, a read-out pulse is applied to a read-out electrode, and at the same time a pulse of a polarity opposite to the read-out pulse is applied to an adjacent electrode and the like, thereby suppressing a rise of the depletion voltage. This will be described with reference to FIGS. 5 to 8.
FIG. 5 is a diagram of a solid-state imaging device of the pixel interpolation type. Plural photodiodes (photoelectric converting elements) 1 are arranged in a checkered pattern on the surface of a semiconductor substrate. The characters R (red), G (green), and B (blue) shown on the photodiodes 1 indicate the colors of color filters stacked on the photodiodes. Photodiode rows on which color filters of G are stacked, and those on which color filters of R and B are alternately stacked are arranged alternately while being shifted by ½ pitch.
The solid-state imaging device is of the pixel interpolation type, and configured so that the interpolation calculation is applied to signals detected by the photodiodes (actual pixels) which are arranged in a checkered pattern, thereby obtaining signals of imaginary pixels at the other checkered positions.
A vertical charge transfer path (VCCD) 2 which meanders so as to avoid photodiodes is disposed on the right next to each photodiode column. A horizontal charge transfer path (HCCD) 3 which communicates with end portions of the vertical charge transfer paths 2 is disposed of a lower edge portion of the semiconductor substrate. An output amplifier 4 is disposed in an output stage of the horizontal charge transfer path 3.
Each of the vertical charge transfer paths 2 is configured by an embedded channel disposed in the semiconductor substrate, and transfer electrode films 2a which are disposed on the channel via a gate insulating film. In the illustrated solid-state imaging device, four vertical transfer electrode films 2a are disposed for each photodiode. The vertical transfer electrode films 2a at the same vertical position of each vertical charge transfer path 2 are electrically connected to one another in the horizontal direction, formed while meandering so as to avoid the photodiodes 1, and subjected to an application of the same pulse. Pulses φV1, φV2, φV3, φV4 are applied to the four vertical transfer electrode films 2a, respectively.
FIG. 6 is a waveform chart of the pulses φV1, φV2, φV3, φV4 for reading out signal charges from the photodiodes by progressive scanning, and transferring them in the vertical direction. In the figure, VH=+15 V, VM=0 V, and VL=−8 V.
FIGS. 7 and 8 are potential transition diagrams of vertical charge transfer paths of first and second lines which are driven by the pulses φV1 to φV4. At time t3, φV1 is set to the VH level (read-out pulse), and a deep potential well is formed (the black arrow a in FIG. 7) under the transfer electrode film in the obliquely right downward direction of the photodiode G (the photodiode 1 on which the color filter of G is stacked) shown in FIG. 5. The signal charge of the photodiode G is read out into the potential well. At this time, the potential of φV3 is set to the opposite polarity, i.e., the VL level, so that the potential at the position of the electrode film is raised as indicated by the black arrows b in FIG. 7, whereby the depletion voltage is prevented from rising.
Thereafter, times t4, t5, t6, t7 elapse. In accordance with the elapse, the potentials of the pulses φV1 to φV4 are changed as shown in FIG. 6. Then, the signal charge of the photodiode G is vertically transferred by a distance which exactly corresponds to two vertical transfer electrode films 2a. 
When φV3 is set to the VH level (read-out pulse) at time t7, deep potential wells are formed (the black arrow c in FIG. 8) under the transfer electrode films in the obliquely right downward direction of the photodiodes R, B shown in FIG. 5. The signal charges of the photodiodes R, B are read out into the potential wells. At this time, φV1 is at the VL level. When the potential of φV2 is set to the opposite polarity, i.e., the VL level, therefore, the potentials at the positions of the electrode films to which the potential φV2 is applied are raised as indicated by the black arrow d in FIG. 8, whereby the depletion voltage is prevented from rising.
In the state of time t7, the transfer position of the signal charge of the photodiode G, and the transfer positions of the signal charges of the photodiodes R, B are aligned with one another at the same vertical position. Thereafter, the transferring operation in the vertical transferring direction is performed.
At time t7 of FIG. 8, the vertical transfer electrode film to which φV3 that is the VH level is applied, and that to which φV2 that is the VL level is applied are at positions adjacent to each other. Therefore, the potential difference of the electrodes is as large as VH−VL=23 V.
In recent solid-state imaging devices, in order to mount several million or more pixels, miniaturization is advancing, and the thickness of a gate insulating film is thinning. Therefore, a large potential difference between adjacent electrode films causes various issues of reliability. For example, leakage vertical lines due to local destruction of a gate insulating film appear in a photographed image, and that, during electric field concentration caused by a large potential difference, the interface level due to impact ions is varied, and dark properties are impaired.